Plastic Injection Molding

Injection Molding produces plastic parts by forcing molten material into a mold where it cools and hardens.  The molded shape produced is a reverse image of the mold tool. Injection molding is low cost for simple and complex parts. Tooling adds to the initial cost but is quickly amortized.

With injection molding, granular plastic is fed by gravity from a hopper into a heated barrel. As the granules are slowly moved forward by a screw-type plunger, the plastic is forced into a heating chamber, where it is melted. As the plunger advances, the melted plastic is forced through a nozzle that rests against the mold, allowing it to enter the mold cavity. The mold remains cold so the plastic solidifies almost as soon as the mold is filled.

Injection molding is an extremely versatile process for producing a wide range of simple or complex plastic parts - economically and with a good finish. Injection molding's efficiency varies by the number of parts you plan to produce. For small quantities it is usually less expensive to simply machine the desired parts. 

Possible shapes

A wide variety of simple or complex 2D or 3D shapes.

Plastic Injection Molding Design Guidelines

• Use an approximately uniform wall thickness throughout your design.

• Keep walls thin - typically between 1/32" and 1/10". This allows for proper cooling and reduces cost by minimizing use of material. Thin walls also reduce problems with material shrinkage. Although some unevenness will occur due to shrinkage, walls as thick as 1/5" can be used. Keep wall thickness at least wall length / 50. Keep 90 deg walls under 0.25" high. Keep thickness of ejection pin surface wall at least .07".

• To strengthen parts, instead of using thicker walls, use additional structures such as ribs. Use fillets at the base of ribs.

• When using a rib make it about half the main wall thickness. 

• Round corners and edges wherever possible.

• For easy release of the part from the mold, add a slight taper to the sides (typically ~ 2 deg) - especially for textured walls and walls higher than 0.25".

Avoid shapes that are impossible to remove from the mold.  Lighter colors hide flow patterns better than dark colors. Choose the right material from the table below. Drawing dimensions should be of the final part - material shrinkage will automatically be considered in the design of the mold. Use raised text instead of recessed text when possible. Where walls meet at a 90 angle, round inside and outside to at least .05" radius - sharper outside corners can create molding problems and sharper inside corners will increase tooling cost. Keep holes at least .015" from edges. It should not be possible to fully hide a 0.3" diameter ball anywhere inside the material.

Example Plastic Injection Molding Parts

Injection molded parts are widely used in: aerospace, automotive, engineering prototypes, hydraulics and pneumatics, packaging, architecture, appliances, fiber optics, medical and dental, power tools, agriculture, electronics, geophysics, measuring instruments, telecommunication, caps, enclosures, valves, toys, levers, cams, etc.

Advantages of Plastic Injection Molding

Low cost and good repeatability. Extremely cost efficient in larger quantities.

Specifications for Plastic Injection Molding

• Material - ABS, acrylic, polycarbonate, high density polyethylene, acetyl, polyamide, low density polyethylene, polypropylene, filled and blended plastics.
• Alternative machines - Mill 3-Axis (for short runs).
• Tooling - custom mold for your design.
• Reducing costs - minimize size and material volume, avoid sharp corners on all features - for example a round post is better than a square post.

Plastic Injection Molding Notes

If desired specify the following:

• Where to place the gate - the location where plastic is injected - a small rough spot will appear at this location.
• Where to place the parting line - the location where the two mold halves meet - a thin line will appear at this location.
• What surface finish to use - polished, matte, textured.

A small slightly rough spot appears at the Gate. A thin line appears at the Parting Line. A circular mark appears at the ejector pin locations.

Living Hinges - A Living Hinge is a thin connection provided between two sections of a molded part so that it can be used as a hinge, e.g. a box, with a lid, molded as one piece. You can use living hinges in your injection molded parts.

Multiple parts in one design - In some cases you can design several different parts in one tool to avoid the cost of making separate molds for each part. Generally the limit is about four if the parts are fairly different and up to 24 if the parts are nearly identical. To have multiple parts in one tool, join the parts together with a narrow connecting channel. If desired, you can "Remove connecting links". Combining parts in one mold does not always decrease the cost - check pricing both ways.

Box seams - Since a seam between two halves of a box is difficult to fully hide, most designers make the seam pronounced - more visible - to make it look like it is decorative. Look at some molded products to see some styles.

Insert Molding - A screw driver with a plastic handle is an example of insert molding. Injection molding is performed around a metal (or another plastic) part. Additional examples include threaded metal inserts and electrical plugs.

Material Selection